In recent years demands for shortwave lasers are increasing in the fields of laser machining, laser medical treatment, material surface reforming and optical information processing, and their vigorous research and development are in progress. Since semiconductor lasers are only capable of generating light from the infrared to blue regions, however, it is essential to adopt a wavelength conversion technique for light ultra-violetization. As such wavelength conversion techniques, SHG (second harmonic generation) and THG (third harmonic generation) utilizing the nonlinear optical effect of an optical crystal are in use. While such crystals as composed of KTP (KTiOPO4) and KDP (KH2PO4) have so far been used as crystal bringing about the nonlinear optical effect, these crystals have problems in that their refractive index fluctuates, their absorption loss is large, they are liable to suffer from optical damages, they are water soluble and hence poor in resistance to environment, their nonlinear optical effect is not much satisfactory and/or their thermal conductivity is poor.
Amongst the attempts to find a nonlinear optical effect crystal that overcomes these shortcomings of KTP and KDP crystals, there has recently been synthesized, as disclosed in JP H10-206916 A, a bulk single crystal called rare-earth/calcium/oxyborate [ReCa4O(BO3)3 where Re is one or more rare-earth elements] for use in SHG (second harmonic generation) and THG (third harmonic generation) of Nd: YAG laser light. Being an oxide optical crystal, this crystal found to get over the difficulties of KTP and KDP crystals and to exhibit extremely high SHG and THG efficiency is considered an optical crystal that should constitute the nucleus of the coming wavelength conversion technique.
However, despite high demand for its application especially in the field of optical information processing, it has so far been unsuccessful to make this crystal in the form of a thin film because of its crystallographic structure which is extremely complicated.
FIG. 6 shows the crystallographic structure of ReCa4O(BO3)3 where Re is one or more rare-earth elements, wherein FIG. 6A is a typical atomic view of this crystal seen from the direction of its b axis, and FIG. 6B is a typical atomic view of the same seen from the direction of its c axis. FIG. 6C shows its basic unit lattice, indicating that this crystal is a monoclinic biaxial crystal belonging to point group m and space group Cm and has its lattice constants a: about 8.09 angstroms, b: about 16. 01 angstroms, and c: about 3.56 angstroms, although precise values vary depending on the type and amount of Re.
As shown, this crystal is extremely large in the number of the atoms making up the basic unit lattice, extremely large in lattice constant and is complicated in structure.
FIG. 7 shows results of the X-ray diffraction measurement of a thin film formed by trying to epitaxially grow ReCa4O(BO3)3 where Re=Gd, Y upon a STO (strontium titanate) and an Al2O3 (alumina) single crystal substrate by laser ablation. As is evident from the diffraction patterns shown, a ReCa4O(BO3)3 thin film epitaxially grown is not obtained.
Since ReCa4O(BO3)3 is extremely complex in the crystallographic structure, epitaxially growing a single crystal thin film thereof upon a substrate requires that the substrate be similar thereto in crystallographic structure and have its surface flattened to an atomic level. Hard to fulfill the requirement of so flattening a surface, the prior art has not been successful in giving rise to such a single crystal thin film.
Thus, the extremely complex crystallographic structure of crystal ReCa4O(BO3)3 has not permitted the state of the art to yield a single crystal thin film thereof and hence to make its nonlinear optical property to be exploited in a thin film structure.
Also, the SHG and THG cannot be carried out efficiently unless the light incident/emitting surface of an optical crystal is flattened to the extent of a wavelength or less. The use in the prior art of an abrasive of a wavelength or less in size to polish such a surface has made it unavoidable that an irregularity in the order of its grain size or a small crack is formed in the surface. As a result, such a surface serving especially to emit high-energy photons may have various optical damages arising from a partial breakage thereof. Preventing such phenomena from taking place requires the structural integrity of the light incident/emitting surface of an optical crystal, which has hitherto been achieved by polishing only to a limited degree, however.
Also, where the quality of an optical crystal is to be assessed, a method may be adopted in which defect portions of the optical crystal are selectively etched and the number of resulting etch pits are counted to assess its quality. If the optical crystal is such an oxide crystal as ReCa4O(BO3)3 which is soluble neither with acid nor with alkaline, the problem is imposed that its quality assessment can by no means be attained conventionally; such an oxide crystal has no means to assess its quality.
With these problems taken into account, it is a first object of the present invention to provide a method of superflattening a surface of an oxide crystal soluble neither with acid nor with alkaline.
Another object of the present invention is to provide a method of making a ReCa4O(BO3)3 family oxide single crystal thin film using such a method and a ReCa4O(BO3)3 family oxide single crystal thin film made thereby.
Further objects of the present invention are to provide a method of superflattening a light incident/emitting surface of an oxide optical crystal and a surface defect assessment method for an oxide crystal in both of which methods the method as the first object is applied.